At the heart of most networks are switches interconnected via a communications medium. For example, Ethernet is a commonly used local area network scheme in which multiple stations are connected to a single shared or dedicated serial data path. These stations communicate with a switch located between the data path and the stations connected to that path. The switch controls the communication of data packets on the network.
Networks are frequently organized into sub-networks, called subnets. Within a single subnet, packets of information may be directed to their destinations using a layer 2 Media Access Control (MAC) address that identifies the attached Ethernet devices. When a switch receives a packet with a familiar destination MAC address, it forwards the packet to the output port on the switch that is associated with the MAC address.
Packets transmitted between layer 2 subnets are forwarded using the destination device's Internet Protocol (IP) layer 3 address. More particularly, a transmitting device sending a packet to a destination device outside of the transmitting device's subnet first determines, using the IP layer 3 address, the layer 2 MAC address of a gateway router that bridges the subnets. The gateway router, upon receiving the packet, performs address translation, which involves stripping the MAC destination address of the router and inserting a new MAC destination address that corresponds to the MAC address of the destination device in the destination subnet. The router determines the MAC address to insert based on the IP address of the destination device.
Network switches may also be used to transmit packets between layer 2 subnets. In these situations, the layer 3 switching operation is commonly performed through the use of a layer 3 internal rules checker (IRC). The layer 3 IRC determines, via an address lookup table, the MAC destination address that corresponds to the MAC address of the destination device in the destination subnet. Not all network switches, however, include a layer 3 IRC. As such, those switches may not be capable of performing layer 3 switching.